Automobile drivers accustomed to driving under snowy or icy conditions are familiar with the technique of "pumping" the brake pedal so as to cyclically increase and decrease the braking force exerted on the wheels such that a slipping wheel having a tendency to lock is permitted to re-accelerate back to speed corresponding to the speed of the vehicle. Such a driver induced anti-skid or antilock technique is simulated by many known anti-skid devices.
An error in, for example, the computation of the wheel speed in such anti-skid or antilock braking systems could be disastrous. An error in wheel speed determination may cause erroneous antilock cycling resulting in increased distance on what should have been a non-antilock stop. In the worst case, should the system falsely detect a wheel as being locked (zero speed), the system could release the braking pressure so that despite operator brake pedal pressure, no braking occurs. One technique for confirming wheel speed computation is disclosed in copending U.S. Ser. No. 07/763,708 entitled "Wheel Speed Verification System" and assigned to the assignee of the present application.
U.S. Pat. No. 4,546,437 addresses the question of reliability of a brake slip control apparatus by providing redundant microprocessors operating under the same software. When inconsistent results are recognized, the brake slip control circuit is disabled and an error signal occurs. A similar approach is utilized in U.S. Pat. No. 4,113,321 wherein a secondary circuit identical to the primary arithmetic circuit confirms operation of the primary circuit. An error when detected renders the anti-skid system inoperable.
It is important to the driver (and required by law) that the driver be aware that the anti-skid system has been rendered inoperable, either due to the detection of such an error or for some other reason, and this is done generally by the energization of a dash panel lamp. For example, in U.S. Pat. No. 3,911,392 a malfunction, such as an interruption of power to any one of a plurality of slip control circuits, is communicated to the driver by rendering a transistor conductive to energize a lamp. The transistor is maintained nonconductive by a set of reverse biased diodes so long as all circuits are receiving power. In somewhat similar fashion, U.S. Pat. No. 3,891,279 teaches a Darlington pair in series with a warning lamp which remain nonconductive so long as an output signal is present on a prescribed terminal of each of several failsafe circuits associated with corresponding skid control circuits.
Current requirements are that the warning indication be energized whenever the anti-skid braking system malfunctions, whenever the connector to the electronic control unit of the anti-skid braking system is disconnected, or whenever electrical power is not available to the anti-skid braking system. The above-noted prior patents fail to provide a warning under one or more of these conditions. Moreover, current techniques for accomplishing such a warning indication in all required instances requires the use of a diode or relay in the wiring harness connected to the anti-skid control circuit. It would be highly desirable that a warning indication be provided in all three cases by the anti-skid control unit and without the need for external circuit components.
Among the several objects of the present invention may be noted the provision of a circuit for communicating any disabling of the anti-skid operation to the vehicle operator; the provision of an anti-skid control unit having a relay-free failure indicator; the provision of a failure indicator for an anti-skid braking system which provides an operator visible warning indication in the event that the anti-skid electronic control unit is disconnected and is otherwise entirely contained within the control unit; and the provision of a simplistic, economical and reliable circuit for advising a vehicle operator of the inoperative status of an anti-skid braking feature. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter.